When a starter is switched on in order to ensure the starting of the thermal engine of the vehicle, a substantial current requirement arises which is close to the short-circuit current level of the starter, i.e. a current of approximately 1000 A. The intensity of this current requirement when the starter is switched on then decreases as the speed of the armature of the starter, corresponding to the rotor of the machine, increases.
A consequent drop in the voltage at the terminals of the battery corresponds to this initial current surge. Other, less substantial voltage drops then occur during the starting phase, and correspond to passages through successive top dead centres of the thermal engine.
The development of so-called “reinforced” starters, suitable for systems for automatic stopping/restarting of the thermal engine (systems known as stop/start or stop and go) now impose new constraints on motor vehicle components manufacturers relating to compliance with minimum voltage thresholds of the battery during the current requirement when the starter is switched on. Thus, in their specifications, motor vehicle manufacturers define a first voltage threshold which is habitually between 7 and 9 V, below which the battery voltage must not drop. For the following voltage drops, corresponding to the top dead centres of the thermal engine, the battery voltage must remain higher than a second voltage threshold which is habitually between 8 and 9 V. During the starting of the thermal engine, the voltage of the vehicle on-board network thus remains at a value which is sufficient to guarantee the required functioning of the vehicle equipment.
Reinforced starters generally have power which is higher than conventional starters, so as to obtain rapid starting for greater comfort of the users. This results in a higher current requirement when switching gone, and thus a first battery voltage drop which goes beyond the habitual values, and with respect to high requirements. This gives rise to a real difficulty for the designer, since, in order to increase the battery voltage, the starter would have to have internal voltage drops which were so high that there would then no longer be the power necessary to drive the thermal engine at a sufficient speed at low temperature.
Solutions have been proposed to the above-described problem in the prior art. A first known solution is based on the use of voltage-increasing electronic converters in order to prevent an excessively low voltage on the on-board network. A major disadvantage of these converters consists in the substantial costs which they introduce.
Another known solution proposes controlling the starter by means of two relays, timing, and current limitation resistance. In a first functioning phase, the duration of which is determined by the timing, additional resistance is inserted in series in the starter circuit, and limits the initial current surge. In a second functioning phase, the additional resistance is taken out of the starter circuit in order to permit the passage of sufficient current in the armature of the starter, and to permit an increase in speed of the latter.
Documents EP2080897A2 and EP2128426A2 describe a starter of the above-described type. As well as the disadvantage of the additional cost which the supplementary control relay, the timing and the current limitation resistance involve, the introduction of this supplementary relay, which involves mobile mechanical parts subject to wear, has a negative impact on the resistance of the starter in terms of the number of starting cycles which the starter must be able to withstand without difficulty. The resistance of the starter in terms of the number of starting cycles is a particularly stringent constraint for starters which are designed for stop/start systems. In fact, such starters are required to withstand approximately 300,000 starting cycles, i.e. 10 times more than the approximately 30,000 cycles required from the conventional starters.
In addition to the above-described disadvantages, the use of this second solution according to the prior art can prove unsuitable when compliance with a voltage range which is restrictive in terms of time is required by the motor vehicle manufacturer.
For the purpose of eliminating the aforementioned disadvantages, the inventive body has already proposed improvements to the existing starters of the prior art, in particular for applications in motor vehicles with the function of stopping and automatic restarting of the thermal engine.
In general, these improvements have consisted of fitting a filtering device of an inductive type in the power circuit of the starter, so as to prevent a drop of the battery voltage after the current surge produced by putting the electric motor into service.
A filtering device of this type described in particular in document PCT/FR2011/052638 comprises a primary winding circuit which is designed to be inserted in series in the said power circuit, and preferably a short-circuited secondary winding circuit. As can be seen in FIG. 1, the primary winding circuit W1 which is arranged around the core C of the filtering device LPF is formed by a conductor 3 in the form of a flattened part wound on the flattened part, i.e. with the smaller side 1 of the flattened part according to the radius and the larger side L of the flattened part according to the height, which causes problems of dimensions. In addition, a configuration of this type imposes substantial clearance according to the height in the axial direction between the winding and the head, taking into account the substantial helix pitch between the turns which can be seen clearly in FIG. 2.